The flow regime in Bhutan is relatively stable in that sense that the year-to-year variations in minimum, average and maximum flow are relatively small compared to what is experienced in many other parts of the world. Furthermore, the fact is that the most of rivers in Bhutan run through steep and narrow gorge rather than e.g. low-lying delta areas, reduces the risk of disasters arising from flood. Also, unlike other countries with very high population density, there are few people who settle along riverbanks in areas subject to regular flooding.

Fig. Hydrograph for Wangdi Rapids for 1994, including the GLOF on 7 October 1994. The hydrograph is based on average daily discharge.

However, still flash flood carrying substantial amounts of boulders and debris and flooding causes substantial damage and loss of lives in Bhutan from time to time.

The most severe floods reported in the past have been glacial lake outburst floods (GLOF) caused by outburst of glacial lakes. However, only three of these floods have been reported and all of them have taken place on the Puna-Tsang Chhu, due to outburst of glacial lakes in the Lunana area (JICA/RGoB, 1999). The most recent one occurred on 7 October 1994 and caused, among other things, substantial damage to the Dzong in Punakha. To provide an idea of the magnitude of the flow related to the glacial lake outburst in 1994 the hydrograph for 1994 for the station at Wangdi Rapids is shown in Figure 3.4, including the GLOF on 7 October 1994.

In the year 2000 flash floods triggered by monsoon rains particularly in the southern Bhutan along the border with India, severe floods were observed at Barsachhu, Dutikhola (small Catchments) and Ammochhu sub-basins, which has cause severe damages to lives and properties.In general, the regions that are most affected by floods in Bhutan are the Phochumochu in the northern part of the country and the Amochu basins in the western part.